The invention relates generally to the field of image sensors and, more particularly, to such image sensors having the edge of a lateral overflow drain self aligned to a gate of the field oxide layer.
Currently known and utilized full frame image sensors, such as in U.S. Pat. No. 5,130,774, include lateral overflow drains for preventing blooming, as is well known in the art. These drains are typically formed underneath the CCD gate electrodes and are limited in performance by surface breakdown. This is because, as the dose of the drain is increased to improve its conductivity, it""s breakdown voltage drops. Therefore, there is a maximum dose and correspondent maximum amount of drain-limited blooming protection that can be provided for a given minimum tolerable breakdown voltage for any given device process.
To avoid this surface breakdown limitation, the drain can be placed underneath the thick field oxide that is typically used for channel-to-channel isolation between the vertical CCDs of these devices as described in U.S. patent application Ser. No. 09/945,034, A LATERAL OVERFLOW DRAIN, ANTI-BLOOMING STRUCTURE FOR CCD DEVICES HAVING IMPROVED BREAKDOWN VOLTAGE, by Edmund K Banghart and Eric G. Stevens. By placing the field oxide layer over the lateral overflow drain, the surface electric field is reduced in inverse proportion to the thickness. In implementing such a device, the lateral overflow drain should be aligned to the edge of the field oxide later. If the drain is not fully covered by the field oxide later, its breakdown voltage will be reduced and limited by the portion of the drain that protrudes out beneath the thinner gate dielectric. If, on the other hand, the drain is placed too far underneath the field oxide layer, connection to it via the buried channel may be lost. This is because the buried channel is typically self-aligned to the field oxide edge by implanting it after the field oxide growth. This may render the structure nonfunctional. Although the latter limitation could be eliminated by implanting the buried channel prior to the growth of the field oxide layer, the former problem would still exist.
Consequently, a need exists for overcoming the above-described shortcomings by providing a process wherein the lateral overflow drain is underneath and self aligned to one edge of the field oxide layer.
The present invention is directed to overcoming one or more of the problems set forth above. Briefly summarized, according to one aspect of the present invention, the invention resides in a method for creating a lateral overflow drain, anti-blooming structure in a charge coupled device, the method comprising the steps of (a) providing a substrate of a first conductivity type for the charge coupled device; (b) providing a layer of oxide abutting the substrate; (c) providing a layer of nitride abutting the oxide; (d) providing a hard mask abutting the nitride with an etched away portion having a dimension which substantially equals a combined dimension of heavily doped first and second conductivity type subsequently implanted regions in the substrate; (e) placing photoresist in a portion of the etched away portion which remaining etched away portion includes a dimension substantially equal to the first conductivity type subsequently implanted region in the substrate; (f) implanting ions of the first conductivity type through the remaining etched away portion and into the substrate for creating a channel stop; (g) removing the photoresist and placing a second photoresist layer in a portion of the etched away portion wherein a remaining etched away portion includes a dimension substantially equal to the second conductivity type subsequently implanted region in the substrate and wherein the remaining etched away portion is adjacent the implanted channel stop; (h) implanting ions of the second conductivity type through the remaining etched away portion and into the substrate for forming the lateral overflow drain; (i) etching a portion of the nitride so that a peripheral portion of the etched away portion is substantially aligned with a peripheral portion of the second conductivity type; (j) growing a thick field oxide in the etched away portion of the nitride layer so that the lateral overflow drain is covered by the thick field oxide layer.
The above and other objects of the present invention will become more apparent when taken in conjunction with the following description and drawings wherein identical reference numerals have been used, where possible, to designate identical elements that are common to the figures.
The present invention has the following advantage of self-aligning a peripheral edge of the lateral overflow drain to an edge of the field oxide, whereby the overflow drain is substantially fully covered by the field oxide.